With the advent of the dawn of the Information Highway and the explosion of telecommunications, the quantity and speed of data transmission continues to grow. In the telecommunications industry, as well as in the computer industry, there exists a need to transmit large quantities of data from point to point, for example between memory and processors in multiple processor computers. The large number of data bits coupled with the large number of connections create an interconnect bottle-neck which requires large numbers of data drivers with their associated large amount of electrical power.
One way that is employed to overcome this congestion difficulty is to multiplex large numbers of parallel bit streams up to higher rate serial bit streams, thus reducing the numbers of electrical connections that need to be made. The need for low power multiplex and demultiplex circuits capable of combining data signals at, say, 50 Mb/s up to, for example, 1 Gb/s has attracted a number of commercial integrated circuit vendors. Nevertheless, the computer and communications industry continues to search for lower power solutions.
A technique that has been employed with success to reduce the number of interconnections in a communications switching equipment is to employ a method known as a contactless backplane, a technique based on directional coupling principles wherein data transfer occurs between proximate conductors. An example of one such coupling connector is described in U.S. Pat. No. 5,432,486 which issued Jul. 11, 1995 to Wong and was assigned to Northern Telecom Limited. Such a method permits point-to-multipoint and multipoint-to-point data transmission over a passive backplane without loss of signal integrity due to the multipoint connections. In this method, distribution of the multi gigabit-per-second serial data employs a form of ac coupling of such small proportions that the data information is contained in the data transitions, thus eliminating the requirement to transmit signal bandwidth below, for example, 1 GHz. In such a methodology, the received data at the demultiplex circuit is considerably attenuated Signal levels of only 70 mV peak to peak, or less, are not uncommon. Reliable reception of this data therefore requires special techniques, including signal amplification, wide frequency bandwidth, matched input impedance and some form of hysteresis to discriminate against unwanted noise signals. The resultant signal must then be restored to NRZ format.
In a co-pending Patent Application entitled "Noise Cancellation Modification to a Non-Contact Bus" by John Williamson et al., in U.S. Pat. No. 6,016,086 which issued Jan. 18, 2000 and assigned to Northern Telecom Limited, a differential microwave coupler is disclosed that achieves ac coupling of considerably attenuated signals similar to those described above. The coupler provides a canceling effect of undesirable data pulse reflections caused by vias, connectors, and other sources of controlled impedance discontinuities. This effect is provided by the configuration of the coupler's inputs, one of which is shorted to ground and the other of which is open circuited. The purpose of this open-short configuration of the inputs is to reverse the polarity of the undesirable reflections at one input with respect to the other input, thereby translating differential reflections into common mode reflections. However, a result of this open-short configuration is that the outputs of the coupler have mismatched dc characteristics.
Other documents of interest in the field of distributed high speed data include two co-pending U.S. Pat. Nos. 5,852,637 and 5,058,144: "Serial Multi-Gb/s Data Receiver" and "Multi-Gb/s Data Pulse Receiver", both by Anthony K. D. Brown and assigned to Northern Telecom Limited. These two documents present a serial multi-Gb/s data receiver, whose characteristics include wide frequency bandwidth, matched input impedance and, in particular, a method for automatic hysteresis adjustment for very small continuous data signals The second disclosure, "Multi-Gb/s Data Pulse Receiver", improves the original receiver as it was not suitable for working in conjunction with a coupler of the type disclosed by Williamson due to dc biasing problems caused by the mismatched dc characteristics of the coupler's outputs. The improved receiver is immune to any dc biasing problems that the coupler may present, as well as provides rejection of any common mode reflections that the coupler introduces.
Standard communications theory states that the input noise level relative to the hysteresis level must be of the order of -24 dB to obtain a transmission bit error rate of one error in 10.sup.-14. The need to adjust the hysteresis level of a receiver in order to obtain this bit error rate requires that the signal level of the data at the receiver be known. The signal level will vary due to variations at the data driver (power supply, process and temperature), as well as due to variations of the transmission medium and attenuation Consequently, it is necessary to continuously monitor the level of the signal at the receiver. The weakness of the multi Gb/s data pulse receiver is therefore that, while it offers automatic hysteresis adjustment for continuous data signals, it cannot support easily hysteresis adjustment for ATM style data.
Asynchronous Transfer Mode (ATM) is a connection-oriented packet switching technique, in which all packets are of fixed length (53 bytes). Generalizing, ATM style data does not necessarily ensure strict conformity with this definition, and may even consist in variable length packets. However, this style of data is bursty in nature, and must be transmitted at its peak rate of burst, with the possibility that the average arrival time between bursts may be quite large and randomly distributed. Due to the intermittent nature of the data, it is difficult to continuously monitor the data and the associated time constants are too long for immediate adjustment of the hysteresis control. Therefore, it is difficult to continuously compensate for signal strength and variation with time and distance.
The background information provided above shows that there exists a need in the industry to provide a data pulse receiver capable of compensating the distributed high speed data signal attenuation incurred over a transmission medium.